The invention is based on an air flow rate meter for determining the mass, or flow rate, of a flowing medium.
U.S. Pat. No. 4,527,423 discloses an air flow rate meter for determining the mass of a flowing medium, in particular of the air aspirated by an internal combustion engine, which comprises a Venturi-like portion of a main flow conduit and an air bypass conduit that discharges into the main flow conduit in the flow direction in a narrowest cross section of the Venturi-like portion. A disadvantage in this arrangement is that the output signal of a temperature-dependent measuring resistor disposed in the air bypass conduit does not correspond, in every engine operating state, to the mass of air actually aspirated by the engine. Deviations from the output signal generated by the measuring resistor from the value corresponding to the air flow rate are caused by pulsations in the air flow when the throttle device is fully opened and by return flow effects of the portion of the aspirated air flowing through the air bypass conduit under certain load conditions. To avoid these measurement errors, an air bypass conduit is proposed in which the ratio of the flow length is 3:1 relative to the flow length of the main flow conduit.
In another known device for determining the mass or flow rate of the aspirated air, in which a temperature-sensitive measuring resistor is also disposed in an air bypass conduit, it has been found that the characteristic curve of the measuring resistor has an unfavorable course in some ranges, with a curvature that does not vary uniformly with an increasing flow rate of the medium. Ranges of constant slope alternate with those of variable slope. This characteristic is the result of a detachment zone that forms directly downstream of the measuring resistor and impedes the flow around the measuring resistor. This detachment zone, in which unsteady flow conditions, characterized by periodic vortex separations at a downstream trailing edge of the measuring resistor, prevail, is generally known as a Karman vortex street. From the negative pressure prevailing in the vortices, a flow that oscillates transversely to the main flow develops in the direction of the negative pressure regions, leading to the aforementioned wrong characteristic curve.
The disturbances in the flow around the measuring resistor that lead to the wrong characteristic curve do not occur in every operating state of the engine in the same way, and this limits the replicability of the characteristic curve of the measuring resistor. The quality of the output side of the measuring resistor is also lessened by signal noise superimposed on the output signal.